Captive breeding of the common shrew, Sorex araneus was conducted between 2003 and 2005. Young specimens were collected from natural populations in September of 2003 and 2004. They were kept under long day conditions (16 hours of daylight) until they reached sexual maturity and were then bred. Eleven out of 18 females gave birth to 17 litters, including 13 (76%) that survived to weaning. The total for all litters was 77 offspring, 54 of which (70%) left the nest. Males born in captivity and kept under variable photoperiod conditions attained sexual maturity (combined testis masses ranged from 167 to 236 mg). These results indicate that maintaining of continuous breeding colony under laboratory conditions of this species should be possible. The main problem remains relatively high animal mortality: 42% of caught shrews died before mating began and 57% of males born in captivity did not survive the acceleration of sexual development.
Somatic chromosomes of 17 northern birch mice, Sicista betulina, originating from lowland and Tatra Mountain populations were studied. In the whole studied material constant diploid number of chromosomes (2n = 32) was found. Polymorphism of a pair of large-sized autosomes was found; acrocentric, subtelocentric and submetacentric chromosomes in homozygous or heterozygous state form the polymorphic pair. Consequently, chromosome arm number (NF) varies within 60 and 62. In this regard Polish populations of northern birch mouse differ from those of more eastward distribution (NF = 63–64).